Thompson equates the political endorsements to what those celebrities do for sneakers, sports drinks and cars.
"They are the perfect Trojan horse," he said. "Cameras tend to be magnetically attracted to them."
A candidate who is seen "as an old foggy" can benefit from Hollywood associations, he said.
During the 1968 presidential race, Richard Nixon made a brief appearance on the popular TV show "Laugh-In," asking "Sock it to me?" Several people have credited that appearance with helping Nixon win the White House.
Bill Clinton started out as "the Hollywood, rock and roll president," Thompson said. "It eventually began to play against him."
Thomspon said Obama already has charisma, is attractive and seen by some as a rock star.
"There's a campaign that doesn't need the kind of things that celebrity can deliver," he said.
Ultimately, however, endorsements don't lead diehard political supporters to switch affiliations.
"In most cases, celebrity endorsements tend to make you change your mind about the celebrity, not the candidate," Thompson said.
New York venture capitalist Alan Patricof, a major Clinton fundraiser, said Democrats and Hollywood stars agree on many issues.
"It seems traditionally they've been more progressive in their thinking and less conservative," Patricof said of the celebrities.
Not all stars are Democrats. Charlton Heston supported several conservative causes and was president of the National Rifle Association. John Wayne was also involved in Republican politics. But for the most part, "celebrity types seem to gravitate toward the ideals of the Democratic Party," Patricof said.
Celebrities aren't working the phone banks making calls for donations. But when they appear at an event, he said, that gives it "star power" and helps draw a larger crowd of donors.
"Their name associated with an event helps sell more tickets," Patricof said. "They're tapping into the traditional network, [which] energizes and mobilizes it. … They're an important element. Without them, some events would be difficult to get off the ground."
While celebrities might be more noticeable these days, they have been part of politics for decades.
Ross said the earliest example he came across in his research is Mable Normand, who backed the Socialist candidate for Los Angeles mayor in 1913. (Normand starred in movies with Charlie Chaplin and has been credited with being the first person to throw a cream pie in films.)
Another early Hollywood player in politics was Louis B. Mayer, co-founder of film studio MGM. He was a head of the California Republican Party and played an active part in Herbert Hoover's presidential campaign.
The 1960s cookbook "Many Happy Returns: The Democrats' Cook Book, or How to Cook a GOP Goose" included an introduction by Frank Sinatra.
"Not only should every Democrat own a copy of this book, but he should load up all his or her friends, and even smuggle some copies into Pasadena and other points where the enemy is strong and square," he wrote.
The cookbook is just one example of how stardom and politics really merged in the 1960s, especially for the Democrats thanks to the campaign of John F. Kennedy.
Today, Democrats and big-name Hollywood stars are very much in line. Conservatives like to point out this alliance when talking about left-wing conspiracies.